Collision possibility acquiring apparatus which detect an obstacle about the own vehicle and determine a collision possibility between the own vehicle and the obstacle have conventionally been known. An example of techniques using such a collision possibility acquiring apparatus is a collision preventing apparatus. When there is a possibility of the own vehicle colliding with an obstacle, for example, the collision preventing apparatus evades the collision by informing the driver of the danger of collision or automatically controlling the own vehicle to decelerate (see, for example, Japanese Patent Application Laid-Open No. 7-104062).